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| United States Patent |
5,738,696
|
|
Wu
|
April 14, 1998
|
Method for making high permeability grinding wheels
Abstract
An efficient method for manufacturing bonded abrasive articles comprises
the use of elongated abrasive grain having a length to cross-sectional
width aspect ratio of at least 5:1 to yield abrasive articles which are
highly permeable to the passage of fluids. A method for measuring
permeability is provided. The abrasive articles are used to carry out soft
grinding and deep cut grinding operations. The permeable abrasive articles
provide an open structure of pores and channels permitting the passage of
fluid through the abrasive article and the removal of swarf from the
workpiece during grinding operations.
| Inventors:
|
Wu; Mianxue (Worcester, MA)
|
| Assignee:
|
Norton Company (Worcester, MA)
|
| Appl. No.:
|
687816 |
| Filed:
|
July 26, 1996 |
| Current U.S. Class: |
51/296; 51/293 |
| Intern'l Class: |
B24D 003/00 |
| Field of Search: |
51/293,296
|
References Cited
U.S. Patent Documents
| 3273984 | Sep., 1966 | Nelson | 51/296.
|
| 3537121 | Nov., 1970 | McAvoy | 51/295.
|
| 3547608 | Dec., 1970 | Kitazawa | 51/295.
|
| 4401442 | Aug., 1983 | Oide | 51/297.
|
| 5009676 | Apr., 1991 | Rue et al. | 51/309.
|
| 5035723 | Jul., 1991 | Kalinowski et al. | 51/309.
|
| 5037452 | Aug., 1991 | Gary et al. | 51/293.
|
| 5129919 | Jul., 1992 | Kalinowski et al. | 51/309.
|
| 5185012 | Feb., 1993 | Kelly | 51/295.
|
| 5203886 | Apr., 1993 | Sheldon et al. | 51/309.
|
| 5221294 | Jun., 1993 | Carman et al. | 51/296.
|
| 5244477 | Sep., 1993 | Rue et al. | 51/293.
|
| 5429648 | Jul., 1995 | Wu | 51/296.
|
| 5431705 | Jul., 1995 | Wood | 51/309.
|
| Foreign Patent Documents |
| 1175665 | Oct., 1984 | CA | .
|
| 86-209880 | Sep., 1986 | JP | .
|
| 91-161273 | Jul., 1991 | JP | .
|
| 91-281174 | Dec., 1991 | JP | .
|
Primary Examiner: Jones; Deborah
Attorney, Agent or Firm: Porter; Mary E.
Claims
I claim:
1. A method for making an abrasive article, comprising abrasive grain and
bond in amounts effective for grinding; comprising the steps
a) blending a mixture comprising abrasive grain consisting of a major
amount of elongated abrasive grain having a length to cross-sectional
width aspect ratio of at least 5:1 and vitrified bond to form an abrasive
mix;
b) pressing the abrasive mix in a mold to form a green abrasive article
having about 55% to about 80%, by volume, porosity; and
c) firing the green abrasive article at 600.degree. to 1300.degree. C. for
a firing time and under firing conditions effective to cure the green
abrasive article and form the abrasive article,
whereby relative to a firing time effective to cure an equivalent green
abrasive article which does not contain the elongated abrasive grain under
the firing conditions of step c), the firing time effective to cure the
green abrasive article is reduced by at least one-half, and whereby the
abrasive article has sufficient interconnected porosity to yield an air
permeability capacity, measured in cc air/second/inch of water, of at
least 0.44 times the cross-sectional width of the abrasive grain.
2. The method of claim 1, whereby the abrasive article following cure has
less than 3%, by volume, variation in size relative to the green abrasive
article, and the green abrasive article is substantially free of
springback following pressing.
3. The method of claim 1 wherein the abrasive article, comprises 60 to 70%
by volume porosity.
4. The method of claim 1, wherein the abrasive article comprises 3 to 15%,
by volume, vitrified bond.
5. The method of claim 1 wherein the abrasive article, comprises 15 to 43%,
by volume, of the elongated abrasive grain.
6. The method of claim 1, wherein the elongated abrasive grain has a length
to diameter aspect ratio of at least 6:1.
7. The method of claim 1, wherein the abrasive article is substantially
free of pore inducer materials.
8. The method of claim 1, wherein the abrasive mix further comprises
materials selected from the group consisting of abrasive grain, filler,
processing aids, combinations thereof, and agglomerates thereof.
9. The method of claim 1, wherein the elongated abrasive grain is sintered
sol gel alpha alumina abrasive grain.
10. The method of claim 8, wherein the filler is selected from the group
consisting of ceramic fiber, glass fiber, organic fiber, combinations
thereof, and agglomerates thereof.
11. The method of claim 6, wherein the article has a permeability of at
least 50 cc/second/inch of water for abrasive grain larger than 80 grit.
12. The method of claim 1, wherein the abrasive article is formed by firing
the green abrasive article at a temperature of about 1100.degree. to
1300.degree. C. for about 1 to 5 hours.
13. The method of claim 9, wherein the abrasive article comprises about 16
to 34%, by volume, of the elongated abrasive grain.
14. The method of claim 1, wherein the abrasive article comprises of about
15 to 55%, by volume, of the elongated abrasive grain and about 5 to 20%,
by volume, bond.
15. A method for making an abrasive article, comprising abrasive grain and
bond in amounts effective for grinding; comprising the steps
a) blending a mixture comprising abrasive grain consisting of a major
amount of elongated abrasive grain having a length to cross-sectional
width aspect ratio of at least 5:1 and vitrified bond to form an abrasive
mix;
b) pressing the abrasive mix in a mold to form a green abrasive article
having about 40% to less than 55%, by volume, porosity; and
c) firing the green abrasive article at 600.degree. to 1300.degree. C. for
a firing time and under firing conditions effective to cure the green
abrasive article and form the abrasive article,
whereby relative to a firing time effective to cure an equivalent green
abrasive article which does not contain the elongated abrasive grain under
the firing conditions of step c), the firing time effective to cure the
green abrasive article is reduced by at least one-half, and whereby the
abrasive article has sufficient interconnected porosity to yield an air
permeability capacity, measured in cc air/second/inch of water, of at
least 0.22 times the cross-sectional width of the abrasive grain.
16. The method of claim 15, whereby the abrasive article following cure has
less than 3%, by volume, variation in size relative to the green abrasive
article, and the green abrasive article is substantially free of
springback following pressing.
17. The method of claim 15, wherein the abrasive article comprises 60 to
70% by volume, porosity.
18. The method of claim 15, wherein the abrasive article comprises 3 to 15%
by volume, vitrified bond.
19. The method of claim 15, wherein the abrasive article comprises 15 to
43%, by volume, of the elongated abrasive grain.
20. The method of claim 15, wherein the elongated abrasive grain has a
length to diameter aspect ratio of at least 6:1.
21. The method of claim 15, wherein the abrasive article is substantially
free of pore inducer materials.
22. The method of claim 15, wherein the abrasive mix further comprises
materials selected from the group consisting of abrasive grain, filler,
processing aids, combinations thereof, and agglomerates thereof.
23. The method of claim 15, wherein the elongated abrasive grain is
sintered sol gel alpha alumina abrasive grain.
24. The method of claim 22, wherein the filler is selected from the group
consisting of ceramic fiber, glass fiber, organic fiber, combinations
thereof, and agglomerates thereof.
25. The method of claim 20, wherein the article has a permeability of at
least 50 cc/second/inch of water for abrasive grain larger than 80 grit.
26. The method of claim 13, wherein the abrasive article is formed by
firing the green abrasive article at a temperature of about 1100.degree.
to 1300.degree. C. for about 1 to 5 hours.
27. The method of claim 13, wherein the abrasive article comprises about 16
to 34%, by volume, of the elongated abrasive grain.
28. The method of claim 15, wherein the abrasive article comprises of about
15 to 55%, by volume, of the elongated abrasive grain and about 5 to 20%,
by volume, bond.
Description
BACKGROUND OF THE INVENTION
The invention relates to a process for making an abrasive article by
utilizing elongated abrasive grains to achieve high-permeability abrasive
articles useful in high-performance grinding applications. The abrasive
articles have unprecedented interconnected porosity, openness and grinding
performance.
Pores, especially those of which are interconnected in an abrasive tool,
play a critical role in two respects. Pores provide access to grinding
fluids, such as coolants for transferring the heat generated during
grinding to keep the grinding environment constantly cool, and lubricants
for reducing the friction between the moving abrasive grains and the
workpiece surface and increasing the ratio of cutting to tribological
effects. The fluids and lubricants minimize the metallurgical damage
(e.g., burn) and maximize the abrasive tool life. This is particularly
important in deep cut and modern precision processes (e.g., creep feed
grinding) for high efficiency grinding where a large amount of material is
removed in one deep grinding pass without sacrificing the accuracy of the
workpiece dimension. It has been discovered that grinding performance
cannot be predicted only on the basis of porosity as a volume percentage
of the abrasive tool. Instead, the structural openness (i.e., the pore
interconnection) of the wheel, quantified by its permeability to fluids
(air, coolant, lubricant, etc.), determines the abrasive tool performance.
Permeability also permits the clearance of material (e.g., metal chips or
swarf) removed from an object being ground. Debris clearance is essential
when the workpiece material being ground is difficult to machine or gummy
(such as aluminum or some alloys), producing long metal chips. Loading of
the grinding surface of the wheel occurs readily and the grinding
operation becomes difficult in the absence of wheel permeability.
To make an abrasive tool meeting porosity requirements, a number of methods
have been tried over the years.
U.S. Pat. No. 5,221,294 of Carman, et al., discloses abrasive wheels having
5-65% void volume achieved by utilizing a one step process in which an
organic pore-forming structure is burnt out during cure to yield a
reticulated abrasive structure.
JP Pat. No.-A-91-161273 of Gotoh, et al., discloses abrasive articles
having large volume pores, each pore having a diameter of 1-10 times the
average diameter of the abrasive grain used in the article. The pores are
created using materials which burn out during cure.
JP Pat. No.-A-91-281174 of Satoh, et al., discloses abrasive articles
having large volume pores, each pore having a diameter of at least 10
times the average diameter of the abrasive grain used in the article. A
porosity of 50% by volume is achieved by burn out of organic pore inducing
materials during cure.
U.S. Pat. No. 5,037,452 of Gary, et al., discloses an index useful to
define the structural strength needed to form very porous wheels.
U.S. Pat. No. 5,203,886 of Sheldon, et al., discloses a combination of
organic pore inducers (e.g., walnut shells) and closed cell pore inducers
(e.g., bubble alumina) useful in making high porosity vitrified bond
abrasive wheels. A "natural or residual porosity" (calculated to be about
28-53%) is described as one part of the total porosity of the abrasive
wheel.
U.S. Pat. No. 5,244,477 of Rue, et al., discloses filamentary abrasive
particles used in conjunction with pore inducers to produce abrasive
articles containing 0-73%, by volume, pores.
U.S. Pat. No. 3,273,984 of Nelson discloses an abrasive article containing
an organic or resinous bond and at least 30%, by volume, abrasive grain,
and, at most, 68%, by volume, porosity.
U.S. Pat. No. 5,429,648 of Wu discloses vitrified abrasive wheels
containing an organic pore inducer which is burned out to form an abrasive
article having 35-65%, by volume, porosity.
These and other, similar efforts fall into two major categories, neither of
which practically meet the requirements for a high permeability abrasive
tool.
The first category is burn-out methods. Pore structure is created by
addition of organic pore inducing media (such as walnut shells) in the
wheel mixing stage. These media thermally decompose upon firing of the
green body of abrasive tool, leaving voids or pores in the cured abrasive
tool. Drawbacks of this method include: moisture absorption during storage
of the pore inducer; mixing inconsistency and mixing separation, partially
due to moisture, and partially due to the density difference between the
abrasive grain and pore inducer; molding thickness growth or "springback"
due to time-dependent strain release on the pore inducer upon unloading
the mold, causing uncontrollable dimension of the abrasive tool;
incompleteness of burn-out of pore inducer or "coring"/"blackening" of an
fired abrasive article if either the heating rate is not slow enough or
the softening point of a vitrified bonding agent is not high enough; and
air borne emissions and odors when the pore inducer is thermally
decomposed, often causing a negative environmental impact.
The second category is the closed cell or bubble method. Introducing
materials, such as bubble alumina, into an abrasive tool induces porosity
without a burnout step. However, the pores created by the bubbles are
internal and closed, so the pore structure is not permeable to the passage
of coolant and lubricant, and the pore size typically is not large enough
for metal chip clearance.
To overcome these drawbacks, and yet preserve and maximize the respective
benefits of each pore inducing method, the invention takes advantage of
the poor packing characteristics of elongated or fiber-like abrasive
grains having a length to diameter aspect ratio (L/D) of at least 5:1 to
increase wheel permeability as well as porosity. Selected fillers, having
a similar filamentary form may be used or in combination with, the
filamentary abrasive grain.
When used in abrasive article compositions, the elongated abrasive grains
yield high-porosity, high-permeability and high-performance abrasive tools
after firing or curing, without the drawbacks of the burn outland pore
inducer methods.
SUMMARY OF THE INVENTION
The invention is a method for making an abrasive article, comprising at
least about 55% to 80%, by volume, interconnected porosity, and abrasive
grain and bond in amounts effective for grinding; comprising the steps
a) blending a mixture comprising elongated abrasive grain having a length
to cross-sectional width aspect ratio of at least 5:1 and vitrified bond
to form an abrasive mix;
b) pressing the abrasive mix in a mold to form a green abrasive article;
and
c) firing the green abrasive article at 600.degree. to 1300.degree. under
conditions effective to cure the green abrasive article and form the
abrasive article,
whereby the firing step is carried out over a period of time which is at
least one-half of the time needed under the same conditions to fire an
equivalent green abrasive article which does not contain the elongated
abrasive grain, and the abrasive article has an air permeability measured
in cc air/second/inch of water of at least 0.44 times the cross-sectional
width of the abrasive grain.
The invention also includes a method for making an abrasive article,
comprising from about 40% to less than 55%, by volume, interconnected
porosity, and abrasive grain and bond in amounts effective for grinding;
comprising the steps
a) blending a mixture comprising elongated abrasive grain having a length
to cross-sectional width aspect ratio of at least 5:1 and vitrified bond
to form an abrasive mix;
b) pressing the abrasive mix in a mold to form a green abrasive article;
and
c) firing the green abrasive article at 600.degree. to 1300.degree. C.
under conditions effective to cure the green abrasive article and form the
abrasive article,
whereby the firing step is carried out over a period of time which is at
least one-half of the time needed under the same conditions to fire an
equivalent green abrasive article which does not contain the elongated
abrasive grain, and the abrasive article has an air permeability measured
in cc air/second/inch of water of at least 0.22 times the cross-sectional
width of the abrasive grain.
By employing this method, the abrasive article following cure has less than
3%, by volume, variation in size relative to the green abrasive article,
and the green abrasive article is substantially free of springback
following pressing.
DETAILED DESCRIPTION OF THE INVENTION
The abrasive article made according to the invention comprises effective
amounts of abrasive grain and bond needed for grinding operations and,
optionally, fillers, lubricants or other components. The abrasive articles
preferably contain the maximum volume of permeable porosity which can be
achieved while retaining sufficient structural strength to withstand
grinding forces. Abrasive articles include tools such as grinding wheels,
hones and wheel segments as well as other forms of bonded abrasive grains
designed to provide abrasion to a workpiece.
The abrasive article may comprise about 40 to 80%, preferably 45 to 75% and
most preferably 50 to 70%, by volume, interconnected porosity.
Interconnected porosity is the porosity of the abrasive article consisting
of the interstices between particles of bonded abrasive grain which are
open to the flow of a fluid.
The balance of the volume, 20 to 60%, is abrasive grain and bond in a
volumetric ratio of about 20:1 to 1:1 grain to bond. These amounts are
effective for grinding, with higher amounts of bond and grain required for
larger abrasive wheels and for formulations containing organic bonds
rather than vitrified bonds. In a preferred embodiment, the abrasive
articles are formed with a vitrified bond and comprise 15 to 40% abrasive
grain and 3 to 15% bond.
In order to exhibit the observed significant improvements in wheel life,
grinding performance and workpiece surface quality, the abrasive articles
made according to the invention must have a minimum permeability capacity
for permitting the free flow of fluid through the abrasive article. As
used herein, the permeability of an abrasive tool is Q/P, where Q means
flow rate expressed as cc of air flow, and P means differential pressure.
Q/P is the pressure differential measured between the abrasive tool
structure and the atmosphere at a given flow rate of a fluid (e.g., air).
This relative permeability Q/P is proportional to the product of the pore
volume and the square of the pore size. Larger pore sizes are preferred.
Pore geometry and abrasive grain size or grit are other factors affecting
Q/P, with larger grit size yielding higher relative permeability. Q/P is
measured using the apparatus and method described in Example 6, below.
Thus, for an abrasive tool having about 55% to 80% porosity in a vitrified
bond, using an abrasive grain grit size of 80 to 120 grit (132-194
micrometers) in cross-sectional width, an air permeability of at least 40
cc/second/inch of water is required to yield the benefits of the
invention. For an abrasive grain grit size greater than 80 grit (194
micrometers), a permeability of at least 50 cc/second/inch of water is
required.
The relationship between permeability and grit size for 55% to 80% porosity
may be expressed by the following equation: minimum
permeability=0.44.times.cross-sectional width of the abrasive grain. A
cross-sectional width of at least 220 grit (70 micrometers) is preferred.
For an abrasive tool having from about 40% to less than about 55% porosity
in a vitrified bond, using an abrasive grain size of 80 to 120 grit
(132-194 micrometers), an air permeability of at least 29 cc/second/inch
of water is required to yield the benefits of the invention. For an
abrasive grit size greater than 80 grit (194 micrometers), a permeability
of at least 42 cc/second/inch of water is required.
The relationship between permeability and grit size for from about 40% to
less than 55% porosity may be expressed by the following equation: minimum
permeability=0.22 .times.cross-sectional width of the abrasive grain.
Similar relative permeability limits for other grit sizes, bond types and
porosity levels may be determined by the practitioner by applying these
relationships and D'Arcy's Law to empirical data for a given type of
abrasive article.
Smaller cross-sectional width grain requires the use of filament spacers
(e.g., bubble alumina) to maintain permeability during molding and firing
steps. Larger grit sizes may be used. The only limitation on increasing
grit size is that the size be appropriate for the workpiece, grinding
machine, wheel composition and geometry, surface finish and other,
variable elements which are selected and implemented by the practitioner
in accordance with the requirements of a particular grinding operation.
The enhanced permeability and improved grinding performance of the
invention results from the creation of a unique, stable, interconnecting
porosity defined by a matrix of fibrous particles ("the fibers"). The
fibers may consist of abrasive grain or a combination of elongated
abrasive grain and fibrous fillers. The fibers are mixed with the bond
components and other abrasive tool components, then pressed and cured or
fired to form the tool.
If the particles are arranged even more loosely by another method, such as
by addition of minor amounts of pore inducer to further separate fiber
grain particles, even higher porosities can be achieved. Upon firing, the
article comprised of organic pore inducer particles may shrink back to
result in an article having a smaller dimension when the pore inducer is
thermally decomposed because the particles have to interconnect for
integrity of the article. Thus, organic pore inducers are most preferably
avoided, and, if used, are limited to less than 5%, by volume, of the
wheel. The shrunk final dimension after firing of the abrasive tool and
the resultant permeability created is a function of the aspect ratio of
the fiber particles. The higher the L/D is, the higher the permeability of
the packed array of fibers can be.
It is believed that elongated grain creates structural anisotropy in the
abrasive wheels and this increases the actual number of cutting points of
the wheels compared with granular abrasive grain. Therefore, the wheels
are sharper. In addition, there are more bond posts created per grain with
an elongated grain. As a result, the bond is stronger and the grain has a
longer useful life. These effects permit the manufacture of higher
porosity, higher permeability wheels, with equal or higher structural
strength with an elongated grain, relative to the same grain type having a
short L/D.
Any abrasive mix formulation may be used in the method of invention to
prepare the abrasive articles herein, provided the mix contains abrasive
grain having an aspect ratio of at least 5:1 , and after forming the
article and firing it, yields an article having the minimum permeability
and interconnected porosity characteristics specified herein.
In a preferred embodiment, the abrasive article comprises a filamentary
abrasive grain particle incorporating sintered sol gel alpha alumina based
polycrystalline abrasive material, preferably having crystallites that are
no larger than 1-2 microns, more preferably less than 0.4 microns in size.
Suitable filamentary grain particles are described in U.S. Pat. Nos.
5,244,477 to Rue, et al.; 5,129,919 to Kalinowski, et al.; 5,035,723 to
Kalinowski, et al.; and 5,009,676 to Rue, et al., which are hereby
incorporated by reference. Other types of polycrystalline alumina abrasive
grain having larger crystallites from which filamentary abrasive grain may
be obtained and used herein are disclosed in, e.g., U.S. Pat. Nos.
4,314,705 to Weitheiser, et al.; and 5,431,705 to Wood, which are hereby
incorporated by reference. Filamentary grain obtained from these sources
preferably has a L/D aspect ratio of at least 5:1, preferably 6:1. Various
filamentary shapes may be used, including, e.g., straight, curved,
corkscrew and bend fibers. In a preferred embodiment, the alumina fibers
are hollow shapes.
Any abrasive grain may be used in the articles of the invention, whether or
not in filamentary form in combination with a major amount of filamentary
grain. Conventional abrasives, including, but not limited to, aluminum
oxide, silicon carbide, zirconia-alumina, garnet and emery may be used in
a grit size of about 0.5 to 5,000 micrometers, preferably about 2 to 200
micrometers. These abrasives and superabrasives may be used in the form of
conventional grit particles or elongated particles having an aspect ratio
of at least 5:1. Superabrasives, including, but not limited to, diamond,
cubic boron nitride and boron suboxide (as described in U.S. Pat. No.
5,135,892, which is hereby incorporated by reference) may be used in the
same grit sizes as conventional abrasive grain.
While any bond normally used in abrasive articles may be employed with the
fibrous particles to form a bonded abrasive article, a vitrified bond is
preferred for structural strength and for precision grinding purposes.
Other bonds known in the art, such as organic, metal and resinous bonds,
together with appropriate curing agents, may be used for, e.g., articles
having an interconnected porosity of about 40 to 70%.
The abrasive article can include other additives, including but not limited
to fillers, preferably as non-spherical shapes, such as filamentary or
matted or agglomerated filamentary particles, lubricants and processing
adjuncts, such as antistatic agents and temporary binding materials for
molding and pressing the articles. As used herein "fillers" excludes pore
inducers of the closed cell and organic materials types. The appropriate
amounts of these optional abrasive mix components can be readily
determined by those skilled in the art.
Suitable fillers include secondary abrasives, solid lubricants, metal
powder or particles, ceramic powders, such as silicon carbides, and other
fillers known in the art.
The abrasive mixture comprising the filamentary material, bond and other
components is mixed and formed using conventional techniques and
equipment. The abrasive article may be formed by cold, warm or hot
pressing or any process known to those skilled in the art. The abrasive
article may be fired by firing processes known in the art and selected for
the type and quantity of bond and other components, provided that, in
general, as the porosity content increases, the firing time and
temperature decreases.
In the method of the invention, for an abrasive wheel comprising (e.g., sol
gel alumina) abrasive grain having an aspect ratio of at least 5:1 in a
vitrified bond, the firing cycle time may be reduced by one-half of the
requirements for the same volume percent interconnected porosity in an
abrasive wheel comprising organic pore inducer and no grain or filler
having an L/D aspect ratio of at least 5:1. In a preferred embodiment, an
abrasive wheel mix comprising, on a volume percentage basis, 30-40% grain
(80-120 grit, 6:1 L/D sol gel alumina) 3-15% vitrified bond, 0-5% fillers
and 0-0.5% processing aids, is blended in a mixer, then discharged into
wheel molds, pressed and then dried at 35% relative humidity and about
43.degree. C. The green pressed wheels are kiln fired by heating for about
4 hours at 1250.degree. C.
This method yields a wheel having a volume percentage porosity equivalent
to that obtained utilizing an equal amount of grain, and 5 to 25%, by
volume of the green wheel, of organic pore inducer, but having a
permeability of 2 to 5 times that of the pore inducer wheel. Such wheels
of the prior art are described in detail in U.S. Pat. No. 5,429,648, which
is hereby incorporated by reference. In addition, the method is completed
at 5 times the rate of the burn out method and in one-half the firing time
(utilizing the same kiln, molds and firing temperatures).
Abrasive articles prepared by this method exhibit improved grinding
performance, especially in creep feed precision grinding. Such abrasive
tools have a longer wheel life, higher G-ratio (ratio of metal removal
rate to wheel wear rate) and lower power draw than similar tools prepared
from the same abrasive mix but having lower porosity and permeability
and/or having the same porosity and lower permeability. The abrasive tools
of the invention also yield a better, smoother workpiece surface than
conventional tools.
EXAMPLE 1
This example demonstrates the manufacture of grinding wheels using long
aspect ratio, seeded sol-gel alumina (TARGA.TM.) grains obtained from
Norton Company (Worcester, Mass.) with an average L/D .sup..about. 7.5,
without added pore inducer. The following Table 1 lists the mixing
formulations:
TABLE 1
______________________________________
Composition of Raw Material Ingredients for Wheels 1-3
Parts by Weight
Ingredient (1) (2) (3)
______________________________________
Abrasive grain* 100 100 100
Pore inducer 0 0 0
Dextrin 3.0 3.0 3.0
Aroma Glue 4.3 2.8 1.8
Ethylene glycol 0.3 0.2 0.2
Vitrified bonding agent
30.1 17.1 8.4
______________________________________
*(120 grit, .about.132 .times. 132 .times. 990 .mu.m)
For each grinding wheel, the mix was prepared according to the above
formulations and sequences in a Hobart.RTM. mixer. Each ingredient was
added sequentially and was mixed with the previous added ingredients for
about 1-2 minutes after each addition. After mixing, the mixed material
was placed into a 7.6 cm (3 inch) or 12.7 cm (5 inch) diameter steel mold
and was cold pressed in a hydraulic molding press for 10-20 seconds
resulting in 1.59 cm (5/8 inch) thick disk-like wheels with a hole of 2.22
cm (7/8 inch). The total volume (diameter, hole and thickness) as-molded
wheel and total weight of ingredients were predetermined by the desired
and calculated final density and porosity of such a grinding wheel upon
firing. After the pressure was removed from the pressed wheels, the wheel
was taken away manually from the mold onto a batt for drying 3-4 hours
before firing in a kiln, at a heating rate of 50.degree. C./hour from
25.degree. C. to the maximum 900.degree. C., where the wheel was held for
8 hours before it was naturally cooled down to room temperature in the
kiln.
The density of the wheel after firing was examined for any deviation from
the calculated density. Porosity was determined from the density
measurements, as the ratio of the densities of abrasive grain and
vitrified bonding agent had been known before batching. The porosities of
three abrasive articles were 51%, 58%, and 62%, by volume, respectively.
EXAMPLE 2
This example illustrates the manufacture of two wheels using TARGA.TM.
grains with an L/D .sup..about. 30, without any pore inducer, for
extremely high porosity grinding wheels.
The following Table 2 list the mixing formulations. After molding and
firing, as in Example 1, vitrified grinding wheels with porosities (4) 77%
and (5) 80%, by volume, were obtained.
TABLE 2
______________________________________
Composition of raw material ingredients for wheels 4-5
Parts by Weight
Ingredient (4) (5)
______________________________________
Abrasive grain* 100 100
Pore inducer 0 0
Dextrin 2.7 2.7
Aroma Glue 3.9 3.4
Ethylene glycol 0.3 0.2
Vitrified bonding agent
38.7 24.2
______________________________________
*(120 grit, .about.135 .times. 80 .times. 3600 .mu.m)
EXAMPLE 3
This example demonstrates that this process can produce commercial scale
abrasive tools, i.e., 500 mm (20 inch) in diameter. Three large wheels
(20.times.1.times.8 inch, or 500.times.25.times.200 mm) were made using
long TARGA.TM. grains having an average L/D .sup..about. 6.14, 5.85, 7.6,
respectively, without added pore inducer, for commercial scale creep-feed
grinding wheels.
The following Table 3 lists the mixing formulations. At molding stage, the
maximum springback was less than 0.2% (or 0.002 inch or 50 .mu.m, compared
to the grain thickness of 194 .mu.m) of the wheel thickness, far below
grinding wheels of the same specifications containing pore inducer. The
molding thickness was very uniform from location to location, not
exceeding 0.4% (or 0.004 inch or 100 .mu.m) for the maximum variation.
After molding, each grinding wheel was lifted by air-ring from the wheel
edge onto a batt for overnight drying in a humidity-controlled room. Each
wheel was fired in a kiln with a heating rate of slight slower than
50.degree. C./hour and holding temperature of 900.degree. C. for 8 hours,
followed by programmed cooling down to room temperature in the kiln.
After firing, these three vitrified grinding wheels were determined to have
porosities: (6) 54%, (7) 54% and (8) 58%, by volume. No cracking was found
in these wheels and the shrinkage from molded volume to fired volume was
equal to or less than observed in commercial grinding wheels made with
bubble alumina to provide porosity to the structure. The maximum
imbalances in these three grinding wheels were 13.6 g (0.48 oz), 7.38 g
(0.26 oz), and 11.08 g (0.39 oz), respectively, i.e., only 0.1%-0.2% of
the total wheel weight. The imbalance data were far below the upper limit
at which a balancing adjustment is needed. These results suggest
significant advantages of the present method in high-porosity wheel
quality consistency in manufacturing relative to conventional wheels.
TABLE 3
______________________________________
Composition of Raw Material Ingredients for Wheels 6-8
Parts by Weight
Ingredient (6) (7) (8)
______________________________________
Abrasive grain* 100 100 100
Pore inducer 0 0 0
Dextrin 4.0 4.5 4.5
Aroma Glue 2.3 3.4 2.4
Ethylene glycol 0.2 0.2 0.2
Vitrified bonding agent
11.5 20.4 12.7
______________________________________
*(80 grit, .about.194 .times. 194 .times. ›194 .times. 6.14! .mu.m)
EXAMPLE 4
(I) Abrasive wheels comprising an equivalent volume percentage open
porosity were manufactured on commercial scale equipment from the
following mixes to compare the productivity of automatic pressing and
molding equipment using mixes containing pore inducer to that of the
invention mixes without pore inducer.
______________________________________
Wheel 9 Mix Formulations
Percent by Weiqht
(A) (B)
Ingredient Invention
Conventional
______________________________________
Abrasive grain* 100 100
Pore inducer (walnut shell)
0 8.0
Dextrin 3.0 3.0
Aroma Glue 0.77 5.97
Ethylene glycol 0 0.2
Water 1.46 0
Drying agent 0.53 0
Vitrified bonding agent
17.91 18.45
______________________________________
*(A) 120 grit, 132 .times. 132 .times. 990 .mu.m.
(B) 50% sol gel alumina 80 grit/50% 38A alumina 80 grit, abrasive grain
obtained from Norton Company, Worcester, MA.
A productivity (rate of wheel production in the molding process per unit of
time) increase of 5 times was observed for the mix of the invention
relative to a conventional mix containing pore inducer. The invention mix
exhibited free flow characteristics permitting automatic pressing
operations. In the absence of pore inducer, the mix of the invention
exhibited no springback after pressing and no coring during firing. The
permeability of the wheels of the invention was 43 cc/second/inch water.
(II) Abrasive wheels comprising an equivalent volume percentage of open
porosity were manufactured from the following mixes to compare the firing
characteristics of mixes containing pore inducer to that of the invention
mixes.
______________________________________
Wheel 10 Mix Formulations
Percent by Weight
(A) (B)
Ingredient Invention
Conventional
______________________________________
Abrasive grain* 100 100
Pore inducer (walnut shell)
0 8.0
Dextrin 2.0 2.0
Aroma Glue 1.83 2.7
Animal Glue 4.1 5.75
Ethylene glycol 0 0.1
Bulking agent (Vinsol .RTM. powder)
0 1.5
Vitrified bonding agent
26.27 26.27
______________________________________
*(A) 80 grit, 194 .times. 194 .times. 1360 .mu.m.
(B) 50% sol gel alumina 36 grit/50% 38A alumina 36 grit, abrasive grain
obtained from Norton Company, Worcester, MA.
The wheels of the invention showed no signs of slumpage, cracking or coring
following firing. Prior to firing, the green, pressed wheels of the
invention had a high permeability of 22 cc/second/inch water, compared to
the green, pressed wheels made from a conventional mix containing pore
inducer which was 5 cc/second/inch water. The high green permeability is
believed to yield a high mass/heat transfer rate during firing, resulting
in a higher heat rate capability for the wheels of the invention relative
to conventional wheels. Firing of the wheels of the invention was
completed in one-half of the time required for conventional wheels
utilizing equivalent heat cycles. The permeability of the fired wheels of
the invention was 45 cc/second/inch water.
EXAMPLE 5
This example demonstrates that high-porosity grinding wheels may be made by
using pre-agglomerated grains. The pre-agglomerated grain was made by a
controlled reduction in the extrusion rate during extrusion of an
elongated grain particle, which caused agglomerates to form prior to
drying the extruded grain.
High-porosity wheels were made as described in Example 1 from agglomerated
and elongated TARGA.TM. grain without using any pore inducer (an average
agglomerate had .sup..about. 5-7 elongated grains, and the average
dimension of each was .sup..about. 194.times.194.times.(194.times.5.96)
.mu.m. The nominal aspect ratio was 5.96, and the LPD was 0.99 g/cc. The
following Table 5 lists the mixing formulations. After molding and firing,
vitrified grinding wheels were made with a porosity of 54%, by volume.
______________________________________
Wheel 11 Mix Formulation
Parts by Weight
______________________________________
Abrasive grain* 100
Pore inducer 0
Dextrin 2.7
Aroma Glue 3.2
Ethylene glycol 2.2
Vitrified bonding agent
20.5
______________________________________
*(agglomerates of 80 grit, .about.194 .times. 194 .times. 1160 .mu.m)
EXAMPLE 6
This example describes the permeability measurement test and demonstrates
that the permeability of abrasive articles can be increased greatly by
using abrasive grains in the form of fibrous particles.
Permeability Test
A quantitative measurement of the openness of porous media by permeability
testing, based on D'Arcy's Law governing the relationship between the flow
rate and pressure on porous media, was used to evaluate wheels. A
non-destructive testing apparatus was constructed. The apparatus consisted
of an air supply, a flowmeter (to measure Q, the inlet air flow rate), a
pressure gauge (to measure change in pressure at various wheel locations)
and a nozzle connected to the air supply for directing the air flow
against various surface locations on the wheel.
An air inlet pressure Po of 1.76 kg/cm.sup.2 (25 psi), inlet air flow rate
Qo of 14 m.sup.3 /hour (500 ft 3/hour) and a probing nozzle size of 2.2 cm
were used in the test. Data points (8-16 per grinding wheel) (i.e., 4-8
per side) were taken to yield an accurate average.
Wheel Measurements
Table 6 shows the comparison of permeability values (Q/P, in cc/sec/inch of
water) of various grinding wheels.
TABLE 6
______________________________________
Wheel Permeability
Permeability
Abrasive Wheel
Porosity Q/P cc/sec/inch H.sub.2 O
Sample (Vol. %) Invention
Control
______________________________________
Example 1
(1) 51 45 23
(2) 58 75 28
(3) 62 98 31
Example 2
(4) 77 225 n/a
(5) 80 280 n/a
Example 3
(6) 54 71 30
(7) 54 74 30
(8) 58 106 34
Example 4
(9) 50 45 22
(10) 47 47 28
Example 5
(11) 54 43 25
______________________________________
Data was standardized by using wheels of at least one-half inch (1.27 cm)
in thickness, typically one inch (2.54 cm) thick. It was not possible to
make wheels to serve as controls for Example 2 because the mix could not
be molded into the high porosity content of the wheels of the invention
(achieved using elongated abrasive grain in an otherwise standard abrasive
mix). The control wheels were made using a 50/50 volume percent mixture of
a 4:1 aspect ratio sol gel alumina abrasive grain with a 1:1 aspect ratio
sol gel or 38A alumina abrasive grain, all obtained from Norton Company,
Worcester, Mass.
Wheel 11 comprised agglomerated elongated abrasive grain, therefore, the
data does not lend itself to a direct comparison with non-agglomerated
elongated grain particles nor to the permeability description provided by
the equation: permeability=0.44.times.cross-sectional width of the
abrasive grain. However, the permeability of the wheel of the invention
compared very favorably to the control and was approximately equal to the
predicted permeability for a wheel containing an otherwise equivalent type
of non-agglomerated elongated grain.
The data show that the wheels made by the process of the invention have
about 2-3 times higher permeability than conventional grinding wheels
having the same porosity.
EXAMPLE 7
This example demonstrates how the L/D aspect ratio of abrasive grain
changes the grinding performance in a creep feed grinding mode. A set of
grinding wheels having 54% porosity and equal amounts of abrasive and
bonding agent, made in a Norton Company manufacturing plant to a diameter
of 50.8.times.2.54.times.20.32 cm (20.times.1.times.8 inch), were selected
for testing, as shown in Table 7, below.
TABLE 7
______________________________________
Properties differences among wheels
Control
Grain Control Elongated
Elongated
Grain.sup.a
Mixture Grain Grain 1 Grain 2
______________________________________
(L/D) 50% 4.2:1 4.2:1 5.8:1 7.6:1
50% 1:1
(vol)
Inducer Type
bubble Piccotac .RTM.
none none
alumina + resin
walnut
shell
Air 19.5 37.6 50.3 55.1
permeability
(cc/sec/inch
H.sub.2 O)
______________________________________
.sup.a All grain was 120 grit seeded sol gel alumina grain obtained from
Norton Company, Worcester, MA.
These wheels were tested for grinding performance. The grinding was carried
out on blocks of 20.32.times.10.66.times.5.33 cm (8.times.4.times.2 inch)
of 4340 steel (Rc 48-52) by a down-cut, non-continuous dress creep feed
operation on a Blohm machine along the longest dimension of the blocks.
The wheel speed was 30.5 meters/sec (6000 S.F.P.M.), the depth of cut was
0.318 cm (0.125 inch) and the table speed was from 19.05 cm/min (7.5
in/min) at an increment of 6.35 cm/min (2.5 inch/min) until workpiece
burn. The grinding performance was greatly improved by using elongated
Targa grains to make abrasive wheels having 54% porosity and an air
permeability of at least about 50 cc/second/inch water. Table 8 summarizes
the results of various grinding aspects. In addition to the benefits of
interconnected porosity, the grinding productivity (characterized by metal
removal rate) and grindability index (G-ratio divided by specific energy)
are both a function of the aspect ratio of abrasive grain: the performance
increases with increasing L/D.
TABLE 8
______________________________________
Grinding differences among 4 wheels
Control
Grinding Grain Control Elongated
Elongated
Parameter Mixture Grain Grain 1 Grain 2
______________________________________
Maximum table
17.5 22.5 25 32.5
speed without
burn
G-ratio @15 25.2 23.4 32.7 37.2
in/min speed
G-ratio @25 burn burn 24.2 31.6
in/min speed
Power @15 22 20.8 18.8 15.7
in/min speed
(HP/in)
Power @25 burn burn 30.6 24.4
in/min speed
(HP/in)
Force F.sub.v @15
250 233 209 176
in/min speed
(lbf/in)
Force F.sub.v @25
burn burn 338 258
in/min speed
(lbf/in)
Grindability
2.12 2.08 3.23 4.42
Index @15
in/min speed
Grindability
burn burn 2.43 4.00
Index @25
in/min speed
______________________________________
Speed in cm/minute is equal to 2.54.times.speed in in/min. Force in Kg/cm
is equal to 5.59.times.force in lbf/in.
Similar grinding performance results were obtained for wheels containing 80
to 120 grit abrasive grain. For the smaller grit sizes, significant
grinding improvements were observed for wheels having a permeability of at
least about 40 cc/second/inch water.
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